always return expression instead of glue

[IndrajeetPatil]

closes #151

[codecov[bot]]

Codecov Report

Merging #152 (a2448d2) into master (f0c1101) will not change coverage. The diff coverage is 100.000%.

@@             Coverage Diff             @@
##             master       #152   +/-   ##
===========================================
  Coverage   100.000%   100.000%           
===========================================
  Files            13         13           
  Lines           516        525    +9     
===========================================
+ Hits            516        525    +9     

Impacted Files	Coverage Δ
R/add_expression_col.R	100.000% <100.000%> (ø)
R/centrality_description.R	100.000% <100.000%> (ø)
R/contingency_table.R	100.000% <100.000%> (ø)
R/tidy_model_expressions.R	100.000% <100.000%> (ø)